Carbon dioxide (CO2) is a chemical molecule consisting of one carbon atom covalently bonded to two oxygen atoms. At atmospheric pressure and temperature, carbon doixode is a colorless, odorless gas that exists naturally as a trace gas in the Earth's atmosphere. It is a fundamental component of the Earth's carbon cycle, with a considerable number of sources, both natural and man-made; moreover, there are a significant number of natural carbon sinks including oceans, peatlands, forests and other biota.

Carbon dioxide s an important greenhouse gas produced by human activities, primarily through the combustion of fossil fuels; however, methane, chlorofluorocarbons and other gases are more potent greenhouse gases. Its concentration in the Earth's atmosphere has risen by more than 35% since the Industrial Revolution. Charles D. Keeling was a pioneer in the monitoring of carbon dioxide concentrations in the atmosphere. Atmospheric mixing ratios for carbon dioxide are now higher than at any time in at least the last 800,000 years, standing at 385 parts per million (ppm) compared to a pre-industrial high of 280 ppm. The current rate of increase is around two ppm per year (see Figure 1).

Sinks of Carbon Dioxide

Carbon dioxide is stored in a number of media including seawater, soils and addition to plantbiomass via photosynthesis. While all of these processes have not been quantified in detail, they represent massive fluxes and sinks for sequestration of carbon.

Sources of Carbon Dioxide

Figure 1: The following graph illustrates the rise in atmospheric carbon dioxide from 1744 to 2005. Note that the increase in carbon dioxide's concentration in the atmosphere has been exponential during the period examined. An extrapolation into the immediate future would suggest continued increases. (Source: PhysicalGeography.net)

Respiration

Respiration, both on land and in the sea, is a key component of the global carbon cycle. On land, an estimated 60 Pg C (60 billion tonnes) is emitted to the atmosphere each year by autotrophic respiration. A similar amount, about 55 Pg C, is emitted as a result of heterotrophic respiration.

In the sea, autotrophic respiration is thought to account for about 58 Pg of the dissolved inorganic carbon in surface waters each year, with the contribution of heterotrophic respiration being 34 Pg C.

Although respiration is a large source of carbon dioxide, it is currently smaller than the amount of CO2 that is removed from the atmosphere annually by phtosynthesis, the biochemical process by which plants and other autotrophic organizms convert carbon dioxide into biomass. Consequently, respiration does not currently represent a net source of carbon doxide to the atmosphere.

Vulcanism

Emissions of CO2 due to volcanic activity, though sometimes large on a local scale, are relatively minor on a global scale, accounting for between 0.02 and 0.05 Pg C per year, or less than 1 percent of yearly human-generated carbon dioxide emissions.

Land-use Change

It is estimated that man-made changes in land-use have, until now, produced a cumulative global loss of carbon from the land of about 200 Pg. Widespread deforestation has been the main source of this loss, estimated to be responsible for nearly 90 percent of losses since the mid-nineteenth century. Losses primarily occur due to the relatively long-term carbon sinks of forests being replaced by agricultural land.

The conversion of land from forested to agricultural land can have a wide range of negative effects as far as greenhouse gas emission is concerned. Soil disturbance and increased rates of decomposition in converted soils can both lead to emission of carbon to the atmosphere, with increased soil erosion and leaching of soil nutrients further reducing the potential for the area to act as a sink for carbon.

Current estimates suggest land-use changes lead to the emission of 1.7 Pg C per year in the tropics, mainly as a result of deforestation, and to a small amount of uptake (about 0.1 Pg C) in temperate and boreal areas - so producing a net source of around 1.6 Pg C per year.

Energy - Stationary Sources

Of the carbon dioxide emissions arising from fossil fuel combustion—up to 6.5 Pg C each year—around 40% is a result of electricity generation, with coal-fired generation being the leading sector. Other stationary sources include industrial (particularly iron and steel manufacture), emissions resulting from oil extraction, refinement and transportation, and domestic and commercial fossil fuel use.

Energy - Mobile Sources

Globally, transport-related emissions of carbon dioxide are growing rapidly. They currently consitute around 24% of anthropogenic CO2 emissions. Road transport dominates these emissions, though off-road, air and marine transport emissions are aslo significant. The use of petroleum as a fossil fuel for transportation dominates carbon dioxide emissions from this source. In 1999, in the U. S., more than 30 percent of fossil fuel-related carbon dioxide emissions were a direct result of transportation. With about two-thirds of this being from gasoline consumption by motor vehicles and the remainder coming from diesel and jet fuel use in lorries and aircraft, respectively.

Industry (non-energy-related)

Carbon dioxide is produced in lime and cement manufacture as a result of the heating of limestone. The final amount of CO2 produced varies depending the type of cement being made. Globally, this source is estimated to amount to 0.2 Pg C emission to the atmosphere each year. Significant carbon dioxide emissions (around 0.25 PgC per year) also result from its use in chemical feedstocks.

Biomass Burning

Though responsible for large CO2 emissions over short time-scales, the net CO2 emissions due to biomass burning are difficult to quantify due to the subsequent uptake of CO2 through regrowth of vegetation. An unsustainable (i.e., not off-set by regrowth) fraction equivalent to about 10% of total emissions is generally assumed biomass used in energy-generation, with this figure being incorporated into the total emissions resulting from land-use change.